tri-amping

[AudioFreak]

I thought that the bottom end of my system (the big one that has caused much controversy on the forum) wouldl put out about 144dB peak. I figured that out using this equasion: (10log W)+speaker sensitivity. I was wrong. I have now learned that every time you double the number of speakers, it goes up 6dB. Each speaker is 96.4dB@1W@1m. There are eight speakers, so this would make 114.4dB@1W@1m right? There is 7200W of power. Plug this into the equasion and I get 153dB (YIKES!).

each speaker is 96.4dB@1W@1M and there are 8 of them.....
add 6 dB each time you double the number of speakers.... so in this case you double 3 times which is 18dB increase.... you then also know that if the total power delivered to the 8 speakers is 1W then each speaker will get 1/8W .... now take 3dB for each time you halve 1W to get 1/8W .... you do this 3 times ... 1W --> 1/2W --> 1/4W --> 1/8W which means each speaker will be down 9dB now add the efficiency of a single driver + the gain you get from having 8 drivers - the loss because each driver sees less of the power ........ 96.4 + 18 - 9 = 105.4dB@1W@1M then add the gain for 7200W which is about 38.6dB which will give a total of 105.4 + 38.6 = 144dB SPL @ 1M for the bass so max output from the total system would be 147dB .... know you also need to allow for @ least 20dB headroom so you would never operate the system above 127dB SPL @ 1M rms continuous.

 

[Kilowatt]

Ok, thanks, I guess my original figuring method worked after all, though it was a little different approach, so it's 147dB peak total system SPL. I still don't know how to divide it up 50%, 15%, 35%.

What exactly is that 20dB headroom thing all about? Is that 127dB figure the average music power? Perhaps that number would go up if it's playing White Zombie, and down if it's playing Simon & Garfunkel. Is that true?

 

[AudioFreak]

Music has big peaks #$%$#$^$% big peaks..... so you've gotta allow for them .... if that 7200W amp clips then those speakers are gone you play the watever @ an average of 20dB below the systems output so that its got some hope of handling the peaks without melting down .... oh i mean clipping.... tho in this case they are probably the same thing of course, some of the time it will be quieter than this also... thats just the average program level.

 

[Kilowatt]

if that 7200W amp clips then those speakers are gone

So it's to allow for peaks. I see. So, why is it that if a 200W amp clips a little, it's no big deal, but if my 7200W amp clips, it catastrophic? Those speakers can handle that 7200W just as well as a smaller set of speakers can handle smaller power levels. Just wondering. I do know that clipping is always hard on speakers, so I would of course want to make sure it doesn't clip ($1400 worth of speakers!), but would it really be that bad?

Maybe I should wire the speakers for lower impedance and make the voltage gain a little lower, this way I could get the full 7200W RMS and have plenty of room for large peaks. It would also make designing the amp a lot easier. What do you think?

Now for the redundant and still unanswered question: what is 35% of 147dB?, what is 15% of 147dB?, how do I figure it out? I am tri-amping, <300Hz gets 50%, 300-1500Hz gets 15%, >1500Hz gets 35%.

 

[AudioFreak]

cause when the 7200W amp clips it will output about 300VDC into those poor speakers ... as for different wiring arrangements, you'll need to work out how many amps the system will draw.... then you've gotta work out how to supply all the current. If you keep your original plans, each of the 1800W Amps will have to supply 25Amps on the peaks and 150V.... now you've gotta work out how to supply each of the 4 big amplifiers with 25Amps that alone is a challenge.

 

[AudioFreak]

Kilowatt.... are those 8 woofers shared between 2 channels ie 4 drivers per channel stereo? if so the maths is different to that which is above.... i redo it if need be

 

[Kilowatt]

You may or may not know that I have changed my design slightly to make it easier for me to design. The amp will have 8 channels, each 900W into 12 ohms. Every other channel will be out of phase, so 1800W into 24 ohms x 4 would also be possible, but I will just hook up one speaker to each of the 8 channels. Each of the + (red) speaker terminals will be labeled either + or - according to their phase to make it easier to hook the speakers up so they run in phase. The output devices will be mounted on opposite sides of 4 big, thick walled pieces of square aluminum tubing that I got from a recycling plant. These will be cooled in the same manner as freezer, with a compressor and liquid/gas refrigerant. It will be very cool (in both ways).

 

[AudioFreak]

OIC ... but that didnt answer the question ... are 4 of those speakers for the left channel of a stereo setup and 4 for the right channel?

 

[Kilowatt]

Yes, 4 for right and 4 for left.

 

[AudioFreak]

ok


well i'll correct my maths...

4 woofers per channel @ 96.4dB@1W@1M each...
so add 12dB because there is 4 drivers....
now each driver will get 1/4 the power so subtract 6dB
so 102.4dB@1W@1M

now each speaker is gonna get 900W so for the 4 speakers that is 3600W .... so add 35.6dB

which gives 138dB @ 1M.............

now that is the bass for 1 channel.......

so total RMS output for 1 channel will be 3dB higher than that or 141dB

and total RMS output for the system will be 3dB higher again which will be 144dB

Less 20dB headroom for a total continuous RMS system output of 124dB SPL @ 1M

 

[Kilowatt]

So it would be 3dB more, or 127dB RMS if the amp were used in mono mode (with the inputs connected)?

 

[AudioFreak]

it would be 3dB more if the left and right bass channel were producing the same material AND the 8 drivers where physically right next to each other. Clearly not practical as you need stereo bass above 100Hz @ least.

124dB SPL @ 1M RMS continuous is the number you'll be aiming for. Maybe less depending on the rest of the system once you've decided what you want.

Now the rest of the maths depends on the material you want to listen to....

 

[Brett]

Hi Kilowatt,

many of the old hands here obviously know about your system , but I don't being relatively new here, so without shooting blind, could you tell me in detail please, what drivers, enclosure design, placement (arraying) and amplifiers you'll be using please. A lot of this doesn't add up to me.

Also, as an FYI, the energy spectrum in many types of music has a breakpoint of about 250Hz. This means that to produse a flat spectrum, the amount of power you will need to produce everything below 250Hz, will be the same as what is required to produce all that above 250Hz.

Cheers

 

[Kilowatt]

Ok, there will be a 7200W stereo amp with 8 900W channels, 4 for right and 4 for left. Each channel will drive an Audiobahn ALUM12X subwoofer (12 ohms) up to about 250 or 300Hz, so that will be half of the total system SPL. The subs will be in two quad boxes. The volume of these boxes can be estimated from the recommended enclosure size for the subs. There will be another xover frequency about 1200Hz. Everything above that will be done with horns, because they are very sensitive, and will be 35% of the total SPL. The rest will have some midrange speakers. I would like to know how many dB the mids and the horns would have to produce to achieve a flat spectrum.

 

[AudioFreak]

your most likely going to need horrendous amounts of acoustic power in the mid range so i'd seriously check out the bigger EV mid drivers

 

[Kilowatt]

First, I agree with most things said, and I think I can figure out my tri-amping. Also, those EV speakers look great-WOW! 150W, 98dB, $160, that's probably what I'll use, but there's some stuff I wonder about:

Going back a little to something that was said earlier. My system that I have now puts out 150W RMS (Bi-amped) into the stock speakers (not the ones I normally use) which are rated 87dB@1W@1m. It can reach average dB levels of about 102dB, commonly peaking at 108dB with the built-in CD player, so I assume it's not clipping. If this was a 7200W system instead of 150W, and the speakers were equally sensitive, it should be a little more than 15dB up from that, or 117dB. Now, we all know that my 7200W amp would actually put out only 50% of the total SPL in a tri-amped system, and the speakers would be vastly more sensitive than 87dB@1W@1m, more like 102.4dB@1W@1M, according to AudioFreak. Dispite all this, AudioFreak, you believe that the system will only put out 124dB total RMS SPL. Something does not compute. I have a friend who reaches that kind of dB levels all the time with about 480W and two speakers with passive xovers and 15" subs. Come on, this is a 7200W RMS amp driving 8 unusually sensitive speakers, and we can add 3dB to that because of the mids and highs, and you think it will only be 124dB?!! What I'm getting too is, it seems to me like 20dB headroom is way too much, and 10 would be more reasonable. Here's why I think 10dB would be OK:

AudioFreak:

cause when the 7200W amp clips it will output about 300VDC into those poor speakers

You say that any clipping at all will instantly destroy the speakers in my system because they would see about 300VDC. Each speaker would see 150VDC for a little (darn little) each half cycle, or about 3 times more than with other typical systems. That sounds bad, I know, but that "DC" would only last for a miniscule amount of time, and about the only stress on the speakers would be mechanical, I don't see where the voice coil would really care. Also, when a normal amp clips, it might be putting about 50VDC on the speakers. 50VDC would be just as harmful to an ordinary sub as 150VDC would be to the highly robust Audiobahns right? 3 times the DC voltage to a speaker that's 3 times tougher. If clipping isn't so serious in other systems, it must not be in this one either. Are you sure that clipping of this amp would be that unusually bad? I haven't heard a good reason why.

 

[AudioFreak]

minor clipping can be dealt with but if the amp is continually clipping thru loud passages then this will kill the drivers...either it will melt the VC's or the cones will end up on the floor (albeit eventually). This is a major problem in pro audio systems and there is often equipment employed to ensure that things dont end in disaster.

Thats what the headroom is about... you said your current setup produces peaks of 108dB well the new system will easily produce peaks of over 140dB which happens to be 8 times louder...

The headroom figure depends on how much the source has been compressed/limitted. A good cd will have between 10dB and 20dB between the average content and the peaks ... if there is less then odds are that the source is over compressed or is of a style that is not very dynamic........

 

[Kilowatt]

How's this? Oh, and it's true that I'll need lots of power for midrange.

136dB peak for the mid and 139dB peak for the treb. 15%, 30%. So, 133dB peak and 136dB peak for each channel. I could do the 133dB of midrange with 3 EVM10's and 300W to each of them per channel. That just made the cost of this thing go throught the roof! I could do the 136dB with 3 horns (I'm not sure who makes them, but I can get them from Allman Music.) and 100W to each of them per channel. This is an example of a young audiophile finding out that big systems are not cheap even if you make them yourself! The whole thing should cost just about $5000 US, which actually isn't bad all things considered.

Also, this whole system would draw a little more power than you can get from an American range outlet (12000W, 240VCT 50A).

 

[Kilowatt]

Is there some kind of a circuit that I can put at the input that will take the flat part off the top of the signal if it clips (i.e., it would cut off the DC part of the signal, on a scope, the signal would have a gap at the top instead of a flat part.)?

 

[AudioFreak]

Every other channel will be out of phase, so 1800W into 24 ohms x 4 would also be possible, but I will just hook up one speaker to each of the 8 channels. Each of the + (red) speaker terminals will be labeled either + or - according to their phase to make it easier to hook the speakers up so they run in phase.

Please explain the intended wiring arrangement in a little more detail as i am struggling to see how all the drivers will run in phase given this scheme.